Hydraulic steering system for articulated vehicle

ABSTRACT

A fully hydraulic steering system for a vehicle having a front frame and a rear frame coupled to each other by an articulating pin, the steering system coupling a steering unit operated by a wheel to steering cylinders, for turning the vehicle body. The steering system comprises: a sensor to detect a turning angle of the steering wheel; a sensor to detect an articulating angle of the vehicle body; a controller to compare outputs from these two sensors to drive a wheel position correcting solenoid valve according to the difference between the outputs; a pilot-type diverting valve to allow wheel position correcting oil to branch from a switch pump circuit to the wheel position correcting solenoid valve; and a safety valve to close a drain port of the pilot type diverting valve only when the driver performs steering with the wheel so that the pilot-type diverting valve feeds correcting oil to the wheel position correcting solenoid valve.

TECHNICAL FIELD

The present invention relates to a pilot type fully hydraulic steeringsystem having a steering device of a vehicle such as a wheel loader,fork lift truck and is capable of preventing the deterioration of asteering operation property of a wheel due to deviation of a wheelposition when a vehicle body travels straight and of correctingdeviation of the wheel position, which is caused by frequent turning ofthe wheel.

BACKGROUND

A conventional pilot type fully hydraulic steering system will bedescribed with reference to FIG. 13.

In the same figure, if a wheel H is turned from a straight position H₂to a left turning position H₁ or to a right turning position H₃, adirectional control valve b is changed over from a straight position b₂to a left turning position b₃ so that an oil under pressure suppliedthrough a hydraulic pump A is supplied to a metering unit c and returnedto the directional control valve b. In the case of left turning, adirectional control valve d of a steering valve is switched over to aleft turning position d₁ so that the oil under pressure from hydraulicpumps M (steering pump) and N (switching pump) is supplied to a headside D_(h) of a left steering cylinder D and a bottom side E_(b) of aright steering cylinder E. In the case of right turning, the oil underpressure is supplied to a bottom side D_(b) of the left steeringcylinder D and a head side E_(h) of the right steering cylinder E sothat vehicle bodies F and G are turned relative to each other, wherebythe steering operation is made. The oil under pressure, which isreturned from left and right steering cylinders D and E, is returned toa tank T through the directional control valve b. Designated at L is aworking machine valve P₁ and P₂, which are denoted at two dotted and onechain line and show that the directional control valve b and themetering unit c are operated by the wheel H.

In the conventional pilot type fully hydraulic steering system, it isnecessary to correct the deviation of the wheel position since theturning angle of the wheel and the turning angle of the vehicle body donot always accord with each other at the same ratio because of leakageof the oil under pressure inside each hydraulic device, indifference ofthe steering valve to a pilot oil and revolution of turning of thesteering wheel.

Accordingly, the present invention is to eliminate the drawbacks of theconventional pilot type fully hydraulic steering system and to provide awheel position correcting device capable of eliminating such a drawbackthat the steering of the wheel is deteriorated due to the gradualdeviation of the wheel position when the vehicle body travels straight.

SUMMARY OF THE INVENTION

To achieve the above object, the present invention provides a wheelposition correcting device for pilot type fully hydraulic steeringsystem provided with a hydraulic circuit to connect a steering unitoperated by a wheel to steering cylinders for turning the vehicle body,characterized in comprising a sensor to detect a turning angle of thewheel, a sensor to detect a turning angle of the vehicle body, acontroller to compare outputs (such as voltage and pulse) from these twosensors to drive a wheel position correcting solenoid valve according tothe difference between the outputs, a pilot type diverting valve toallow wheel position correcting oil to branch from a switch pump circuitto a wheel position correcting solenoid valve, and a safety valve toclose a drain port of the pilot type diverting valve only when thedriver performs steering with the wheel so that the pilot type divertingvalve feeds correcting oil to the correcting valve.

With such an arrangement, it is possible to correct the deviation of thewheel position by supplying a wheel position correcting oil from thewheel position correcting solenoid valve to the steering cylinder towardthe position for correcting the deviation of the wheel or by drainingpart of the oil discharged from the steering valve if there is deviationbetween the target turning angle of the vehicle body when it travelsstraight and the estimate turning angle of the same when it returns totravel straight in case that wheel is changed to turn in full range ofthe turning angle of the vehicle body (ranging from the leftward maximumturning angle to the rightward maximum turning angle).

Furthermore, according to the present device, although the wheelposition correcting oil is supplied to pipings between the steering unitand the steering cylinders for correcting the deviation of the wheelwhen the steering valve is indifferent to the pilot oil flow, thisdevice can assure the safety by providing a safety valve (correctingcircuit closing valve) which stops the wheel position correcting oil bya flow divider to prevent the drawback caused by the continuous supplyof the correcting oil when the steering wheel is not turned in case ofthe trouble of the controller or the wheel position correcting solenoidvalve.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a view explaining a pilot type fully hydraulic steeringsystem provided with a wheel position correcting device according to thepresent invention,

FIG. 1B is a fragmentation perspective view of the correcting circuitclosing valve according to the present invention,

FIG. 2 is a view explaining the system according to another embodimentof the present invention,

FIGS. 3(a) and (b) are flowcharts showing an operation of a controller,

FIGS. 4 to 9 are views of assistance in explaining operations of thecontroller as illustrated in the flowcharts in FIGS. 3(a) and (b),

FIG. 10 is a view explaining that a turning direction of a wheel is thesame as the correcting direction thereof (correcting oil is added to anoil flow from the steering valve to the steering cylinders),

FIG. 11 is a view showing that the turning direction of the wheel isreversed to the correcting direction (part of the oil flow from thesteering valve to the steering cylinders is drained),

FIG. 12 is a schematic view showing the correction of changing to turnand

FIG. 13 is a view showing a conventional pilot fully hydraulic steeringsystem.

DETAILED DESCRIPTION

The present invention will be described with reference to drawings.

FIG. 1 is a view explaining a pilot type fully hydraulic steering systemprovided with a wheel position correcting device according to thepresent invention in which the same devices as those as illustrated inFIG. 13 are denoted at the same numerals and detailed explanationsthereof are omitted.

In FIG. 1, a wheel H, a directional control valve 10, a metering unit 20(a steering unit 30 is a general name of a combination of thedirectional control valve 10 and the metering unit 20), a correctingcircuit closing valve 40 are connected by two rows of two dotted onechain lines X₁, X₂, X₃, X₄,X₅ X₆, and X₇ and respectively operated bythe wheel H.

Designated at 50 is a wheel position correcting solenoid valve, 60 is apilot type diverting valve, 90₁, 90₂, 91, . . . , 100 are oil pipings,101 is a controller, 102, 103, 104, 105 are electric wirings, 110 is asensor to detect a turning angle of the wheel and 120 is a sensor todetect a turning angle of the vehicle body.

An operation of the correcting circuit closing valve 40 will bedescribed first. The correcting circuit closing valve 40 supplies awheel position correcting oil to the wheel position correcting solenoidvalve 50 only when the driver performs steering with the wheel, therebycorrecting the wheel position. When the wheel is not turned, the wheelposition correcting oil is not supplied. Accordingly, it is possible toprevent the vehicle body from turning against the intention of thedriver.

More in detail, when the wheel H is not turned, a spool 40₁ of thecorrecting circuit closing valve 40 and sleeves 40₂, 40₃, 40₄ arerespectively centered by springs disposed at both ends of the sleeveswhile the oil under pressure supplied from the pilot type divertingvalve 60 through a pilot hydraulic circuit 92 of the pilot typediverting valve 60 is drained from a small hole of the spool 40₁ and asmall hole of the sleeve 40₃ so that spools 60₁ and 60₂ of the pilottype diverting valve 60 are positioned at the position of the spool 60₁by the oil under pressure from a hydraulic piping 90₁. Consequently,substantially all of the oil under pressure discharged from the pump Aexcepting the small amount of oil which is drained from the pilothydraulic circuit 92 are supplied to a steering oil flow control valve31 through a hydraulic piping 97. Accordingly, the wheel position is notcorrected at this time even if the wheel position correcting solenoidvalve 50 is positioned at an R correction position 50₁ or an Lcorrection position 50₃.

That is, it is possible to perform steering with the wheel even if it istroubled while the wheel position correcting solenoid valve 50 ispositioned at the R correction position 50₁ or the L correction position50₃.

An operation of the correcting circuit will be described with referenceto control flowcharts of a controller 101 as illustrate in FIGS. 3(a)and (b).

Step 1: (refer to FIG. 4) A turning angle θ_(w) of the wheel is detectedby the sensor 110 and the detected value thereof is supplied to thecontroller 101.

Step 2: (refer to FIG. 5) A turning angle θ_(F) of the vehicle body isdetected by the sensor 120 and supplied to the controller 101.

Step 3: The controller 101 calculates an angular velocity θ_(w) of thewheel and angular velocity θ_(F) of the vehicle body is calculated.

Step 4: The controller 101 judges that the turning angle θ_(F) is withinthe straight travelling value of the vehicle body or not (assuming thatjudging value representing that the vehicle body travels straight or notis denoted at ±α, turning left is denoted at + and turning right isdenoted at -). If the expression of α≧θ_(F) ≧-α, is established, thecontroller 101 judges that the vehicle body travels straight and theprogram goes to Step 5.

If the expression of θ_(F) >α or the expression of θ_(F) <-α isestablished, the program goes to Step 8.

Step 5: The controller 101 judges that the vehicle body starts to turnor not (assuming that judging value representing that the vehicle bodyturns or not is denoted at ±β, turning left is denoted at + and turningright is denoted at -).

If the expression of β≧θ_(F) ≧-β, is established, the controller 101judges the possibility of helm angle for course keeping, the programgoes to Step 6.

If the expression of θ_(F) >β or of θ_(F) <-β, is established, thecontroller 101 judges that the wheel is not in the helm angle butturned, the program goes to Step 7-4.

Step 6: The controller 101 judges that the turning angle of the wheel atthe helm angle is within allowable deviation value or not (deviationturning angle of the wheel is ±δ , leftward deviation is denoted at +,rightward deviation is denoted at -).

If the expression of Δθ_(F) >δ, is established, the program goes to Step7-1.

If the expression of Δθ_(F) <-δ, is established, the program goes toStep 7-2.

If the expression of δ≧Δθ_(F) ≧-δ, is established, the program goes toStep 7-3.

The expression of Δθ_(F) =θ_(F) -kθ_(wm) is established, where θ_(wm) isa mean value during time t (=Δt x n) sec. Δt is a sampling time (referto FIG. 6).

Step 7-1: L correction (the wheel position correcting solenoid valve isswitched over to turn the vehicle body leftward and the wheel positioncorrecting oil is supplied).

Step 7-2: R correction (the wheel position correcting solenoid valve isswitched over to turn the vehicle body rightward and the wheel positioncorrecting oil is supplied).

Step 7-3: Correction is not made.

Step 7-4: A pulse counter is reset and set the start turning angleθ_(wo) of the wheel to 0.

The above is the helm angle correction.

A correction of change to turn will be described hereinafter.

Step 8: The controller 101 judges that the wheel is changed to turn ornot. If the expression of θ_(F) >α and of θ_(w) <-γ are established, orif the expression of θ_(F) <-α and of θ_(w) >γ are established, thecontroller 101 judges that the wheel is now changing to turn, theprogram goes to Step 9.

If the expression of α>θ_(F) >-α or γ>θ_(w) >-γ is established, theprogram goes to Step 13-4.

Step 9: The controller 101 calculates the Time T_(F) required for thevehicle body to return to the straight travelling, which is obtained bythe following expression. ##EQU1## Step 10: The controller 101 estimatesthe turning angle θ_(wt) of the wheel when the vehicle body returned tothe straight travelling.

    θ.sub.wt =θ.sub.w +θ.sub.w xT.sub.F

(where the symbols θ_(w) and θ_(w) are reversed to each other at thetime of changing to turn)

Step 11: The controller 101 selects the turning angle of the wheel whereθwt is the closest (an integral multiple of 360°). For example, theturning angle of the wheel is stored in a memory by 360° and the turningangle which is closest to θ_(wt) is selected and assumed to be θ_(wto).

Step 12: The controller 101 compares θ_(wt) and θ_(wto) and judges thenecessity of correction and the direction of the correction. If theexpression of θ_(wt) >θ_(wto) +δ is established at the time of changingto turn the left or the expression of θ_(wt) <θ_(wto) is established atthe time of changing to turn the right, the program goes to Step 13-1.

If the expression of θ_(wt) <θ_(wto) -δ is established at the time ofchanging to turn the left or the expression of θ_(wt) <θ_(wto) +δ isestablished at the time of changing to turn the right, the program goesto Step 13-2.

If the expression of θ_(wto) +δ≧θ_(wt) ≧θ_(wto) -δ is established, theprogram goes to Step 13-3.

Step 13-1: Part of the steering oil flow (discharged oil flow from thesteering valve) is drained from the wheel position correcting valve 50so that the amount of changing to turn the wheel is increased and keptto change to turn (the wheel is more turned compared with the amount ofturning of the vehicle body) (refer to FIG. 7).

Step 13-2 (refer to FIG. 10). The wheel position correcting oil flow ismerged with the steering oil flow through the wheel position correctingsolenoid valve so that the amount of turning of the vehicle body isincreased compared with the amount of changing to turn the wheel (thewheel is less turned compared with the amount of turning of the vehiclebody) (refer to FIG. 8).

Step 13-3: Correction is not made (refer to FIG. 9).

Step 13-4: Correction is not made.

An arrangement of the wheel position correcting device as shown in FIG.2 is different from that of the embodiment as shown in FIG. 1, whereinthe pilot type diverting valve 60 in FIG. 1 is replaced by the pilottype check valve 41 so that the correcting oil is supplied between thesteering unit and the steering pilot port, thereby controlling the pilotoil. Accordingly, the wheel position correcting solenoid valve becomescompact so as to be adapted for the pilot type check valve through whichsmall amount of oil flows with low pressure.

INDUSTRIAL UTILIZATION

As mentioned above, according to the present invention, it is possibleto solve the problem in that the operation property of the wheel isdeteriorated because of the gradual deviation of the wheel positionduring the straight traveling of the vehicle body. In case of frequentturning of the wheel rightward and leftward, it is possible to solve theproblem in that the state where the wheel position is deviated lastssince the turning angle of the wheel is always compared with the turningangle of the vehicle body and corrected in accordance with the amount ofdeviation when the wheel position is deviated. Furthermore, thedeviation correcting operation does not hinder the normal steeringoperation.

Since the correcting circuit closing valve is structured by the oil holeprovided at the steering unit, there are such a prominent effect that anadditional space is unnecessitated and the correcting circuit closingvalve is made at low cost with high reliability.

Since the correcting circuit closing valve can mechanically control theamount of the correcting oil by the driver's steering operation, it ispossible to prevent the correcting oil from supplying continuously intothe steering cylinders and prevent the vehicle body from beingspontaneously turned in case of trouble of the wheel position correctingvalve.

We claim:
 1. A fully hydraulic steering system for an articulatedvehicle having wheeled front and rear frames coupled to each other by anarticulating pin, said steering system including a steering unitoperated by a steering wheel and operatively coupled to sidewardlydisposed first and second hydraulic steering cylinders each having afirst end coupled to said front frame and a second end coupled to saidrear frame for articulating the front and rear frames relative to oneanother about the articulating pin, said steering system comprising:afirst sensor means for detecting a turning angle of the steering wheel;a second sensor means for detecting an articulating angle between thefront and rear frames of the vehicle; a controller coupled to the firstand second sensor means for comparing the outputs thereof, saidcontroller including means for signalling when the articulating angledeviates from a predetermined value relative to the turning angle of thesteering wheel; a steering wheel position correcting valve means forsupplying wheel position correcting oil to the first and second steeringcylinders in response to said signalling means; a steering flow controlvalve means for supplying main steering oil to the first and secondsteering cylinders in response to the steering unit; a pilot-typediverting valve means for supplying hydraulic oil from a switch pumpcircuit to either the steering flow control valve means or the steeringwheel position correcting valve means; and a correcting circuit closingvalve means mechanically coupled to said steering unit for permittingsaid diverting valve means to supply the hydraulic oil to said steeringwheel position correcting valve means only when said steering wheel isoperated.
 2. A fully hydraulic steering system as claimed in claim 1,wherein said correcting circuit closing valve means provides a path fordraining a pilot oil from said pilot-type diverting valve means to asump tank when said steering wheel is not being operated to prevent saidpilot-type diverting valve means from supplying the hydraulic oil tosaid steering wheel position correcting valve means.
 3. A fullyhydraulic steering system as claimed in claim 1, wherein said correctingcircuit closing valve means blocks a path for draining a pilot oil fromsaid pilot-type diverting valve means to a sump tank when said steeringwheel is being operated to permit said pilot-type diverting valve meansto supply the hydraulic oil to said steering wheel position correctingvalve means.